Semiconductor dice, i.e., integrated circuit chips, are commonly packaged within a protective molding compound, such as plastic or epoxy. The semiconductor dice are thereby given a “packaged” structural form that is more easily handled. The molding compound also provides physical and environmental protection to the semiconductor dice. The protective molding compound is typically formed about a die through a transfer molding process where the die is immersed in a hot, liquid form of the molding compound.
After the transfer molding process, the packaged semiconductor device is allowed to cool down so that the molding compound can solidify. The molding compound lowers in temperature and shrinks in size during the cooling process. Unfortunately, the molding compound and the die usually have differing coefficients of thermal expansion and modulus of elasticity. Therefore, the compound and the die will shrink to different extents. As a result, the die will absorb stress and become compressed and deformed by the compound, which causes micro changes in the surface area of the die. Due to the tight requirements on product performance, e.g. output voltage and resistivity, the micro change in the surface area of the die actually alters the product performance such that the performance falls below an acceptable level. Specifically, the change in the size of a resistive area of an integrated circuit can change the desired resistance or output voltage. Also, it is common that the die attach pad, which supports the die, also has a unique coefficient of thermal expansion. As a result, the molding compound and the die attach pad can act together to deform the die.
The differing reactions to heat are also problematic during normal operation of the packaged semiconductor devices since the heat created by the die causes expansion and contraction of the various packaged semiconductor device components. In many cases, the molding compound is formed of a thermoset resin that does not flex when subjected to changing temperatures. Unfortunately, a molding compound that does not flex causes the electrical components within to be subject to an even larger amount of stress.
One current solution involves adjusting the composition of the molding compound. For example, filler material can be added into the molding compound to affect the amount that the compound expands and contracts. The filler material can be formed of silica, which does not have an elastic nature, as it is a brittle material like glass and therefore unable to be bend or deform when in solid form. The size and amount of the silica filler material can be adjusted accordingly. Unfortunately, these solutions only have a limited extent of effectiveness in reducing the stress imposed upon integrated circuit dice. Also, the inhomogeneous nature of the silica distribution within a molding compound induces large variations in a die's electrical performance.
In view of the foregoing, there are continuing efforts to provide improved techniques for reducing stresses imposed upon integrated circuit dice within packaged semiconductor devices.